Blindness due to corneal causes has long been a challenge to ophthalmic surgeons treating the anterior segment of the eye. In many cases, the blindness-causing injury has altered the optical clarity and refractive abilities of the cornea to such an extent that the body cannot satisfactorily repair it, but has left the eye's light perception and projection relatively intact. In such cases, the only alternatives for improving vision are a corneal transplant (keratoplasty) or a prosthesis. Corneas exhibiting severe scarring or vascularization resulting from chemical burns are often irreparable through keratoplasty, whether or not anterior segment reconstruction is performed. Indeed, in these instances and where there is a history of repeated keratoplasty failure because of the inability of the recipient's body to tolerate or provide adequate nutrition to the graft, or where there is the presence of keratitis sicca or pemphigoid, use of a keratoprosthesis is the only alternative for improving vision in the eye.
Investigators in this field have been working for more than a century on the development of simple 1ens keratoprostheses for replacing and functioning as part of the cornea. See, Cardona, "Keratoprosthesis", American Journal of Ophthalmology, Vol. 54, 284-294 (1962). Typically, these keratoprostheses are implanted in the corneal stroma or sutured to it; hence, the only supporting structure for the keratoprosthesis is the cornea itself. Any intraocular portion of the prosthesis is thus left to hang, unsupported, in the anterior chamber.
Complications encountered with presently used keratoprostheses include extrusion of the implant following a progressive erosion or aseptic necrosis of the corneal tissue surrounding the implant. This, in part, may be caused by the stresses placed on the surrounding tissue by the movement of an unsupported implant. Another complication encountered is the tendency for corneal membrane tissue to overgrow the intraocular portion of the prosthesis, thereby interfering with light perception and altering light projection. This tissue overgrowth can, as well, lead to implant extrusion. Additional problems encountered in the use of keratoprostheses presently used are irritation and ulceration of the surrounding tissues which, too, eventually lead to extrusion of the implant. Even in some of the most impressive clinical studies involving keratoprostheses, these complications have resulted in extrusion failure rates generally over 10%. See, Cardona, "Mushroom Transcorneal Keratoprosthesis", American Journal of Ophthalmology, Vol. 68, No. 4, 604-612 (October 1969) and Castroviejo, et al., "Present Status of Prosthokeratoplasty", American Journal of Ophthalmology, Vol. 68, No. 4, 613-625 (October 1969).
Another limitation of presently used keratoprostheses is the fact that the only optically functional lens portion of the prosthesis is permanently fixed in the anterior chamber. Once the keratoprosthesis is implanted, therefore, minor refractive errors of the eye must be corrected by eyeglasses. There exist no available keratoprostheses in which adjustments of the optical characteristics of the lens portion may be accomplished after implantation without invasive surgical techniques. Furthermore, none exist which would permit an entirely new optical lens to be utilized without such procedures. Any surgical intrusion for this purpose carries with it the risks accompanying the loss of intraocular pressure and is to be avoided where possible.